Cellular communication systems are known. As subscriber penetration continues to increase in analog cellular systems, so does the need for increased capacity. Cell splitting to reduce cell coverage while increasing reuse of radio channels will allow more subscribers to be served per unit area. When cells are reduced in size to the microcellular range (i.e. 100-500 meter cell radius), the ability to handover from one cell to another is complicated by the ability of the system to recognize the need to handover and to determine where to handover in a timely fashion (the term handover is also used to mean handoff as is understood in the art). In order to avoid unnecessary dropped calls due to the speed of the vehicle or the street corner effect, handover detection and execution needs to occur within 2-3 seconds. Current analog systems can require up to 8-10 seconds (or greater) to detect the need to handover, find a suitable target with strength measurement requests (HOMRs), and then execute the handover.
One method to counteract this problem was coined Cell Assisted Handover (CAHO), and a U.S. Pat. No. 5,203,010 issued April, 1993. The CAHO method is triggered when the source cell scan receiver detects that the uplink signal has fallen below a threshold. A HOMR is sent to neighbor cells along with the measured signal level and a forecasted signal level. The neighbor cell(s) then measure the mobile signal level and respond if acceptable. If not acceptable, then the neighbor cell will scan this mobile for a fixed window of time to see if the measured signal becomes acceptable with respect to the forecasted signal level. If it does, then the target cell will respond to the source indicating that it is now ready to accept the handover. Then, a final signal level measurement is performed by the source cell to make sure target signal level measurement is still better than source signal level measurement. While this method will tend to reduce the repetitions of HOMRs that don't initially succeed, it does not necessarily improve the overall reaction time to detect and execute a handover. A better method is still needed.
Another problem occurs when a mobile scans signaling channels in its idle mode scanning and locks onto a cell which is at a very great distance from the mobile. This can occur when the mobile is in an elevated position relative to the surrounding area causing line of site propagation with the remote site. When a call is established and the mobile moves from this elevated position, the call will typically drop because the neighbor cells cannot locate the mobile. If the mobile remains in the elevated position, then it can be a significant interferer to the reusers of that frequency.